Boron trifluoride-methanol complex—mild and powerful reagent for deprotection of labile acetylated amines

A set of amino-group possessing cyanine dyes is obtained from their N-acetyl derivatives via deprotection with boron trifluoride-methanol complex in good yields.     

Density functional calculation of the electronegativity and other related properties of atoms and ions of the principal groups of the periodic table

The relation between the electronegativityχ of an atom or an ion (χ=??E(Z,N)/?N) and its finite difference (Mulliken like) counterpart has been studied for the elements of the groups IA to VIIA of the Periodic Table, using an approximate Density Functional Theory. Only the valence electrons are taken into account and the effect of the ionic core is simulated by a simple empty core pseudopotential. The first derivative ?χ/6N of the electronegativity has also been computed. The interest inχ and?χ/?N is illustrated by a simple model for the transfer of electronic charge in a molecule. Molecular electronegativities are then computed.     

Transformation of dynamic DSC results into isothermal data for the curing kinetics study of the resol resins

Kinetics of thermosetting polymers curing is difficult to study by isothermal methods based on the differential scanning calorimetry (DSC) technique. The difficulty is due to the low sensitivity of the equipment for total reaction heat measurements during high temperature process. The aim of this paper is to display the equivalence between a dynamic model, the Ozawa method, and an isothermal isoconversional fit, which allows predicting the isothermal behavior of the resol resins cure through dynamic runs by DSC. In this work, lignin–phenol–formaldehyde and commercial phenol–formaldehyde resol resins were employed. In addition, the isothermal kinetic parameters for both resins were performed by means of transformation of the data obtained from the dynamic Ozawa method.     

Regioselective synthesis of 4- and 5-oxazole-phosphine oxides and -phosphonates from 2H-azirines and acyl chlorides

A simple and efficient regioselective synthesis of 4-oxazole-phosphine oxides 11 and -phosphonates 12 from 2H-azirine-phosphine oxides 1 and -phosphonates 6 is described. The key step for the synthesis of oxazoles 11 is a base-mediated ring closure of vinylogous α-aminophosphorus compounds derived from phosphine oxides 4 and from phosphonates 8. These derivatives 4 and 8 are obtained by reaction of functionalized azirines 1 and 6 with acyl chlorides 2 and subsequent acid-catalyzed ring opening of N-acylaziridine-phosphine oxides 3 and -phosphonates 7. Regioselective thermal ring cleavage of N-acylaziridine-phosphine oxides 3 leads α-chloro-β-(N-acylamido)-phosphine oxides 13 and their treatment with bases gives 5-oxazole-phosphine oxides 16.     

Asymmetric synthesis of 2H-azirines derived from phosphine oxides using solid-supported amines. Ring opening of azirines with carboxylic acids

A simple and efficient asymmetric synthesis of 2H-azirine-2-phosphine oxides 3 is described. The key step is a solid-phase bound achiral or chiral amine-mediated Neber reaction of beta-ketoxime tosylates derived from phosphine oxides 1. Reaction of 2H-azirines 3 and 11 with carboxylic acids 4 gives phosphorylated ketamides 5 and 12. Ring closure of ketamides 5 and 12 with triphenylphosphine and hexachloroethane in the presence of triethylamine leads to the formation of phosphorylated oxazoles 8 and 13.     

All-solid-state potentiometric sensors sensitive to nonionic surfactants based on ionophores containing ethoxylate units

Earlier work of potentiometric Ion-selective electrodes (ISEs) sensitive to nonionic surfactants of the polyethoxylate type is further extended. The ISEs constructed were all-solid-state sensors with plasticized PVC membranes. The sensing material was a tetraphenylborate salt of the barium complex with a polyethoxylate nonionic surfactant. As membrane component, the combinations of two polyethoxylates of the nonylphenoxy type, which differed in the number of oxyethylene units (5 or 12), and two different plasticizers, (o-nitrophenyloctyl ether and o-nitrophenylphenyl ether), were tested. The response of these electrodes to different nonionic surfactants and the interference effect of several species has been evaluated. For all the types of tested electrodes, the sensitivities shown were ca. 30.0 mV dec(-1) and the limit of detection, ca. 10(-5) M, when a nonylphenoxyde with 12 oxyethylene units was used as standard. The membrane with the best response characteristics was then applied in potentiometric titrations of this kind of surfactants in the presence of Ba(2+) ion and using tetraphenylborate as the titrant.     

Genomic clustering of the Trypanosoma cruzi nonlong terminal L1Tc retrotransposon with defined interspersed repeated DNA elements

We have analyzed the genomic distribution and organization of the long interspersed nucleotide element (LINE) L1Tc, a nonlong terminal repeat (LTR) retrotransposon of Trypanosoma cruzi. The results indicate that the L1Tc element is dispersed along the parasite genome and that in some regions it is organized in tandem repeats. The data allowed us to define the existence of short direct-repeated sequences flanking the genomic L1Tc elements. Relevant is the finding that the LINE L1Tc is located in genomic regions rich in short interspersed nucleotide elements (SINE)-like sequences. In particular, the L1Tc element is found associated to E13-related sequences, redefined in this work and renamed RS13Tc, and to a newly described RS1Tc sequence. The RS1Tc sequence is present, per haploid genome, in about 3,200 copies. Northern blot analysis showed that the RS1Tc is being transcribed into RNAs of different sizes. The analysis of the chromosomal distribution of these elements in various strains of T. cruzi suggested that this type of clustering might be a common feature of the genome of these parasites.     

Stability study and determination of benzene- and naphthalenesulfonates following an on-line solid-phase extraction method using the new programmable field extraction system

Seven benzene- and naphthalenesulfonates (3-nitrobenzenesulfonate, 4-methylbenzenesulfonate, 1-hydroxy-4-naphthalenesulfonate, 1-amino-7-naphthalenesulfonate, 4-chlorobenzenesulfonate, 1-naphthalenesulfonate and 2-naphthalenesulfonate) were studied. A rapid method for quantifying aromatic sulfonated compounds from waste water samples was developed. This method consists in on-line in-field sampling and monitoring based on ion-pair solid-phase extraction with PLRP-S sorbent, using the new programmable field extraction system and ion-pair liquid chromatography with UV diode-array and electrospray mass spectrometry. Limits of detection for the studied compounds, using the SIM acquisition mode, ranged from 0.01 to 0.33 ng ml(-1). The influence of the aqueous matrix on the on-line SPE was checked by spiking ground and waste waters. Recoveries varied from 70 to 99% when 10 ml of water sample were enriched. The method was applied to the analysis of some environmental sewage samples. This study confirmed that high concentration levels of aromatic sulfonated compounds can be found in sewage samples. In addition, the stability of the seven studied sulfonated benzene and naphthalene compounds was investigated using on-line polymeric SPE pre-columns, based on the styrene-divinylbenzene polymer PLRP-S. Different storage conditions were tested to carry out the stability survey, which included storage at room temperature, at 4 degrees C and at -20 degrees C, during a period of up to 2 weeks. This study showed that the stability of aromatic sulfonic acids on disposable on-line SPE polymeric pre-columns is related to temperature and that the target compounds are more stable at lower temperatures.     

Latex Fractionation by Sedimentation and Colloidal Crystallization: The Case of Poly(styrene-co-acrylamide)

Poly(styrene-co-acrylamide) (PS-AAM) latex was prepared, fractionated by sedimentation under gravity, and characterized by PCS, infrared spectra, secondary and backscattered electron imaging in the scanning electron microscope, and electron spectroscopy imaging in an analytical transmission electron microscope. Three latex fractions were obtained. The lower fraction was opalescent and its particles were the more uniform, concerning size, chemical composition, and topochemical features. This lower fraction was still further fractionated by zonal centrifugation in a density gradient, yielding two fractions with similar macrocrystal-forming abilities but different sizes and chemical compositions. These results confirm those previously obtained for the PS-HEMA latex. Copyright 2000 Academic Press.